SWRO Desalination: A Viable, Long-Term Solution to Water Scarcity

By G.G. Pique

Oct 13, 2010

Twenty years ago, I was engaged in a pioneering attempt to drought-proof the City of Santa Barbara, Calif., by building a series of trailer-mounted emergency desalination plants. I also have been directly involved in the design of hundreds of desalination plants operating worldwide. Since that time, the water supply/demand equation has shifted dramatically with a profound impact on municipal water rates. The evolution of technology also has increased the affordability of desalination. Given these factors, it is important to understand the implications of vanishing affordable water on humankind and how new technologies are being applied to desalination to help meet water needs in an environmentally friendly manner.

While many of those suffering from water insufficiencies reside in developing countries, dwindling supply impacts a number of industrialized nations, including the United States. Years of drought and mismanagement of resources have led to serious concerns in California where, up and down the coast, water rates have tripled, and rationing and other extreme conservation efforts have been implemented. For instance, in Monterey County, Calif., residents were recently allocated 40 gallons of water per person per day – a very strict restriction. If they conserve and stay within this allocation, they pay approximately $0.98 per cubic meter; however, if they use more than the 40 gallons per day ration, their water rates increase incrementally up to a whopping $9.98 per cubic meter. This is before the customer surcharges planned for the various desalination plants.

Down the coast of California to the large metropolitan areas supplied by the Colorado River, the situation is potentially more critical. Despite higher than normal precipitation this past winter, according to the Bureau of Reclamation data, the Colorado River at Lake Mead has been dropping at an average rate of one foot per month since 2000 to its lowest level in 46 years. If the water level at Lake Mead drops another 8 feet (a situation expected as early as next summer), Arizona and Nevada will be forced to ration their water. A drop of another 25 feet from that point will put the waterbody at a minimum operating point for the electrical generators, potentially creating a much broader power issue.

Below is data tracked by the U.S. Bureau of Reclamation for the Lower Colorado Region, which covers nearly 202,000 square miles and encompasses parts of five states that contribute water to or draw water from the Colorado River: southern California; southern Nevada; the southwest corner of Utah; most of Arizona and part of west-central New Mexico. The recent recordings show that the Colorado River Basin has an alarmingly low supply. This can be linked to an increase in population, increases in withdrawals from the five states and lower precipitation levels.

In recent years, seawater reverse osmosis (SWRO) desalination — the process of extracting fresh water from ocean water using semi-permeable membrane technology — has emerged as a long-term, affordable solution.

Beginning 10 years ago, countries around the world, including Algeria, Australia, China, India, Israel, Spain and even Mexico, adopted SWRO desalination to provide a reliable, drought-proof supply of fresh water. Spain adopted SWRO on a large scale beginning in 1985 to supply growing tourist areas of the Canary Islands and the Costa del Sol region. Other tourist islands in the Caribbean followed suit in the 1990s. In some places and more than 20 years ago, the cost of drinking water produced by desalination plants was 10 to 20 times what people were paying in places like New York.

Here in the United States, desalination has become a controversial, hot-button issue that has sparked intense debate throughout California, along the Gulf Coast and in other areas where water scarcity persists.

Opponents, such as Surf Riders and other environmental groups, maintain that the desalination process causes irreparable environmental damage to marine life and uses far too much energy compared to conservation and wastewater recycling. These are the same claims that have plagued desalination since the 1970s – back when these assertions carried some merit. In its infancy, desalination consumed four to 10 times more energy than it does today and was very expensive (see graph).

Today is a new age in water desalination – one that is affordable and energy efficient. The increasing affordability of desalination is due to the evolution of key technologies (membranes, pumps and energy recovery devices) and their becoming more efficient over time, as well as the fact that, as traditional water sources are becoming scarcer, water rates have been rapidly increasing.

Debunking Desalination MythsBasic SWRO desalination involves pumping ocean water at very high pressures – up to approximately 1,000 psi (40 times the pressure in your car tires) –across membranes to produce potable water, discharging concentrated brine at high pressure back into the ocean. Traditional SWRO systems use large and energy-intensive high-pressure pumps for the job.

Energy efficient technology addresses the power issue today. Isobaric energy recovery devices (ERDs) harness the pressure of the reject stream and recycle it into the SWRO process before the incoming sea water is pressurized. As a result, the pumps do not need to work as hard and, therefore, consume far less energy.

Through precise engineering, an efficiency of 98 percent can be achieved with state-of-the-art isobaric ERDs and, as a result, the technology has attained near universal acceptance within SWRO desalination plants around the world. At this optimal efficiency, the ERD can reduce the amount of energy required to desalinate seawater by up to 60 percent, resulting in more economical production of drinking water and a reduced carbon footprint.

To address concerns about the potential environmental impact to marine life from the concentrate discharges into the ocean, most desalination plants around the world discharge the seawater concentrate very far out to sea in open currents to ensure zero environmental impact. One very effective method is used in Barcelona, where the brine is diluted with pure treated water from the adjacent wastewater treatment plant before being discharged back into the sea. The majority of the plants in Spain and Australia also use diffusers to ensure rapid mixing of the salty brine with seawater to minimize impact.

Another concern is the potential impact of the seawater intake. These systems are designed to minimize the entrainment of solids and marine life that must be removed by the pre-treatment system before the water flows to the SWRO process. The trick is to slow the intake down so that the fish can swim away. Low-impact beach wells and ocean-floor subsurface intakes also are used in Monterey Bay and in Alicante, Spain.

With water scarcity continuing to plague various regions of the United States, it is time the country caught up to the rest of the world. Some progress has been made — the Monterey County town of Seaside, Calif., recently activated its first SWRO desalination facility at Sand City and is providing desalinated drinking water to residents.

But this is just one plant and, as desalination is considered by other communities throughout the Golden State, the opposition will surely continue its assault, trying desperately to inject as much fear, uncertainty and doubt into public perception as possible.

According to Global Water Intelligence, California has 19 desalination plants planned by 2016 to produce an estimated 1.2 million cubic meters of fresh water per day. Many of these projects are in various stages: progress is being made to conduct pilot plants, environmental feasibility studies, and public awareness campaigns.

If decisions on water desalination are based on facts and not the passionate pleas of people who are of goodwill but otherwise misinformed, SWRO desalination plants can provide people with a limitless supply of clean, fresh water using less energy than it takes to pump water through California’s State Water Project aqueduct from Northern California all the way to Los Angeles.